Use of full-tensor dielectric and conductivity measurements on whole core samples, core plugs, and flow-line measurements are increasingly being utilized in the estimation of directional resistivities, anisotropy ratios, fluid saturations, bed boundaries, dips, and other resistivity parameters in geological samples.
Full-Tensor Complex Conductivity EM Tool for Core Analysis
Core analysis is one the ways of evaluating oil-wells. Geological cores are obtained as whole cores or side-wall cores. The assessment of formation characteristics acquired from geological cores is crucial to the decision-making process concerning development plans for petroleum wells that are being evaluated for an exploration or production activity. Full-tensor complex conductivity measurements on geological cores provide valuable directional information about the resistivity properties of the formations, reduces uncertainty in resistivity interpretation from well logs, is non-invasive and non-contact method for resistivity measurements unlike galvanic methods, generates continuous as-received resistivity logs of whole cores, and is the most-suitable core-based resistivity measurement for geologically-complex anisotropic and heterogeneous reservoirs.
In one full-tensor inductive conductivity tool for flow-line measurements and whole core analysis, an alternating current driven through transmitter induces eddy currents in the surrounding volume, which further induce a complex voltage in the receiver coil. Induced voltage is measured using electronics (for example, lock-in amplifiers, etc.) with high signal-to-noise ratio. Moreover, the direct coupling between the transmitter and receiver coils is removed using a bucking coil, which exploits additive nature of the induced voltage responses. The coil configuration in the tool is designed to generate three-dimensional orthogonal magnetic dipoles using one axial coil and two saddle coil arrangement for each of the transmitter, receiver, and buck coils. Such an orthogonal system is capable of measuring tensor inductive conductivity of the formation. Basic 1D-inversion of this conductivity tensor measurement gives horizontal resistivity, anisotropy ratio, dip, and azimuth of the volume of investigation. Also, an emerging technique is to incorporate tilted antennas. In order to use such tools for commercial core analysis application, a standardized petrophysically-consistent calibration technique needs to be developed.
Calibration Techniques for Downhole EM Logging Tools
Tool calibration is an important and necessary task of logging operations. Factors such as imperfection in tool construction and variations due to the tool's electronics can introduce errors in measurements. Methods are available for calibrating EM logging tools to remove such errors in tools having radially outward-looking antennas of a downhole EM tool. These methods use electronic circuits/devices for calibration purposes that are not representative of the petrophysical properties of geological samples.